Pred­at­ory bac­teria

Most people think of mi­croor­gan­isms as a cause of dis­ease rather than as its vic­tims. But in fact, they too can be­come vic­tims of bac­teria that make them sick and even de­vour them. Such a pred­at­ory bac­terium has now been de­scribed by re­searcher Jens Harder and his team from the Max Planck Institute for Marine Microbiology in Bremen, Germany.

The assailant: Undercover for a long time

For more than twenty years, the pred­at­ory bac­teria have been liv­ing largely un­noticed in Jens Harder's labor­at­ory at the Max Planck In­sti­tute in Bre­men in a so-called en­rich­ment cul­ture that uses li­monene for meth­ane pro­duc­tion. They ori­gin­ally came from the di­ges­tion tower of the sewage treat­ment plant in Os­ter­holz-Scharm­beck, close to Bre­men. “We have named the new mi­crobe Velamenicoccus archaeovorus,” says Harder. “It is an ul­tra­mic­robac­terium –a par­tic­u­larly tiny mem­ber of the mi­cro­bial world, only 200 to 300 nano­metres in size and thus in­vis­ible un­der a nor­mal mi­cro­scope.” By com­par­ison, a hu­man is al­most two bil­lion nano­metres tall. A few secrets of these tiny bac­teria have now been re­vealed.

The victim: Important for biogas production

The second main char­ac­ter of this story, the vic­tim, is also found in sewage treat­ment plants: Methanosaeta, one of the most com­mon mi­crobes in the world, plays a cru­cial role therein. This ar­chaeon is mainly re­spons­ible for bio­gas pro­duc­tion in sewage treat­ment plants. In­di­vidual cells of Methanosaeta live to­gether in a pro­tect­ive tube, a fil­a­ment. Us­ing spe­cial dyes and a spe­cial mi­cro­scope, Harder and his team were now able to prove that single cells in the Methanosaeta-fil­a­ments were sick or dead. They were limp and con­tained neither ri­bosomal nuc­leic acids nor ge­netic ma­ter­ial – typ­ical com­pon­ents of liv­ing mi­cro­bial cells. The cells had pre­sum­ably fallen vic­tim to the ul­tra­mic­robac­teria: “Most prob­ably, the cause of dis­ease is at­tached bac­teria, and these at­tached bac­teria are Velamenicoccus archaeovorus,” Harder ex­plains.

The weapon? A giant protein

“Velamenicoccus archaeovorus is not an un­known,” Harder con­tin­ues. “We have found parts of its ge­netic ma­ter­ial in deep sed­i­ments and other oxy­gen-free hab­it­ats. But what it does there was not known.” Now, re­search­ers at the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy have been able to de­code the gen­ome of this ul­tra­mic­robac­terium and identify its pro­teins, thereby un­lock­ing some of the tiny pred­at­or's secrets. One par­tic­u­larly re­mark­able gene is a strik­ingly large one. “While pro­teins on av­er­age con­sist of 333 amino acids, this gene en­codes a pro­tein with 39678 amino acids,” Harder ex­plains. Thereby, it would be one of the largest known pro­teins. It is in­teg­rated into the cell wall and its sur­face con­tains en­zyme do­mains that en­able it to dis­solve cells. Thus, this could well be the deadly secret of Velamenicoccus.

Ecologically significant in deep sediments?

Real­ising that we are deal­ing with such a “dan­ger­ous” bac­terium per­mits a new look at an eco­lo­gical ques­tion: Sed­i­ments are full of mi­croor­gan­isms, de­creas­ing in num­ber with in­creas­ing depth. The deeper you go, the fewer cells you will find. Un­til now, it was as­sumed that this was the res­ult of the on­go­ing die-off of cells. Now an­other pos­sib­il­ity arises: Mi­croor­gan­isms use other mi­croor­gan­isms as a food source, and be­cause this is not par­tic­u­larly ef­fi­cient, the or­ganic ma­ter­ial is be­ing in­creas­ingly lost as meth­ane and car­bon di­ox­ide. “Ul­tra­mic­robac­teria can thus play a de­cis­ive role in the con­ver­sion and re­cyc­ling of bio­mass in sed­i­ments and cause an over­all re­duc­tion in bio­mass with depth,” Harder con­cludes.

Fit­tingly: Velamenicoccus archaeovorus, the ar­chaea-eat­ing mi­crobe, be­longs to the candidate phylum Om­ni­troph­ica, mean­ing the “all-eat­ers”. The find­ing that they live as pred­at­ors now shows for the first time that this ap­pel­la­tion is really ac­cur­ate.